In the manufacture of GaAs FET's intended for use as power FET's at high frequencies, problems are encountered, generally in one or more of device breakdown, insufficient transconductance, substrate leakage, and short channel effect. Additionally, operational failures and/or limitations are encountered as a result of excessive capacitance, excessive source resistance, and insufficient current carrying capacity.
Prior efforts to overcome these difficulties have recognized that parasitic substrate current is a significant factor in high frequency applications, "Substrate Current in GaAs MESFET's", L. F. Eastman and M. S. Shur, IEEE Transactions on Electron Devices, Vol. ED-26, No. 9, Sept. 1979, pp. 1959-61. This publication mentions the use of undoped Al.sub.x Ga.sub.1-x As buffer layers in order to utilize the heterojunction barrier and reduced saturation velocity to provide reduced parasitic conduction.
Kim et al, "Microwave Power GaAs MISFET's with Undoped AlGaAs as An Insulator", IEEE Electron Device Letters, Vol. EDL-5, No. 11, Nov. 1984, pp. 494-495, describes the output power limitations of a MESFET as being related to the gate-drain breakdown voltage and the conduction current through the channel. Use of an insulating or semi-insulating buffer layer is described to increase breakdown while maintaining the channel current level. This can be done in spite of the inverse relation between breakdown voltage and the product of doping level times active layer thickness. It is noted, however, that this approach was not satisfactory and that a HIGFET approach was attempted and rejected due to very low current levels and high parasitic resistances. Finally, a MISFET approach is described wherein an undoped Al.sub.x Ga.sub.1-x As layer is provided over a highly doped GaAs channel. The layers were formed by molecular beam epitaxy (MBE).
Inomata et al, "Improved Transconductance of AlGaAs/GaAs Heterostructure FET with Si-Doped Channel", Japanese Journal of Applied Physics, Vol. 25, No. 9, Sept. 1986, pp. L731-L733 describes HEMT using AlGaAs/GaAs heterostructure and their limitations due to high source resistance. Provision of a doped AlGaAs layer over doped and undoped GaAs channel layers was studied.
Hida et al, "A High-Current Drivability i-AlGaAs/n-GaAs Doped-Channel MIS-Like FET (DMT)", IEEE Electron Device Letters, Vol. EDL-7, No. 11, Nov. 1986, pp. 625-626 describes the problems facing those designing GaAs devices for high speed, high power applications. Among the problems are the need for high current handling capability with large average transconductance for large input signal, high breakdown voltage, good current linearity and high cut-off frequency. It is noted that MESFET's and two dimensional electron gas FET's (2 DEG FET's or HEMT's) cannot adequately meet these objectives. MESFET's are limited because channel electron density cannot exceed donor density and the high field at the gate lowers the breakdown voltage. 2 DEG FET's have low carrier density (about 10.sup.12 cm.sup.-2) resulting in parallel conduction in the n-AlGaAs layer leading to transconductance compression. Additionally, 2 DEG FET's have low breakdown because of the doped channel layer under the gate. These problems were addressed by employing an undoped AlGaAs layer over the doped GaAs channel to provide high carrier density in the GaAs channel and high breakdown due to the undoped AlGaAs adjacent the gate. The layers were all grown by MBE.
Notwithstanding these various attempts to provide GaAs FET's suitable for use as high frequency power FET's, there is a continuing desire to obtain higher power at given high frequencies. In addition to the desire to extend the operating limits of power FET's, it is of significant interest to find a economical and repeatable process for their manufacture. Existing processing techniques rely on MBE for the growth of the various doped layers of high frequency power FET's, including growth of such layers as the doped AlGaAs (HEMT) and doped GaAs (DMT) layers. It has been found that individual devices may be fabricated in a laboratory through such a process but that full scale production using such a process is neither economical nor repeatable.
It is therefore an object of the present invention to provide an improved FET suitable for high-frequency, high-power applications.
It is another object of the present invention to provide a manufacturing process which is repeatable and economical and which provides high frequency power FET's.
These and other objects of the invention are obtained through the provision of a first carrier confining layer below the channel layer with or without a second carrier confining layer above the channel layer, combined with a self aligned gate process. Manufacturability is achieved by abandoning the fabrication of doped layers in favor of fabricating undoped layers with subsequent doping. According to a preferred manner of making the improved FET's, a layer of undoped AlGaAs is formed over a substrate by MBE. Undoped layers of GaAs and AlGaAs are sequentially grown, followed by ion implantation, into the GaAs, of a channel dopant, preferably silicon. Due to the poor activation efficiency of silicon into AlGaAs, the AlGaAs layers are not effectively doped. Thus, due to the thin channel layer and the carrier confinement caused by the AlGaAs sandwiching of the channel, a highly doped channel with high carrier concentration is provided. Also, carrier injection into the substrate is avoided and a high breakdown immunity is obtained. The present invention provides an improved FET design and a production-worthy process for its manufacture.